The English scientist John Dalton (1766-1844) made the
observation which has come to be known as "Dalton's Law" and which
forms the basis for decompression theory. In simplified form, Dalton's Law
states,

The pressure exerted by a mixture of gases is equal to
the sum of the pressures of each of the different gases making up the
mixture--each gas acting as if it alone were present and occupied the total
volume.